Yugi Muto (武む藤とう 遊ゆう戯ぎ Mutō Yūgi), also romanized as Yugi Mutou, is the main protagonist of the original Yu-Gi-Oh! series, along with Yami Yugi. It was stated once that Yugi is the modern day version of Pharaoh Atem.[8] Together with the spirit of Pharaoh Atem, he holds the title "King of Games" (King of Duelists in Japanese version). Yugi possessed the three thousand-year-old (five thousand in the dub) mystical Millennium Puzzle, which allowed his body to bear host to the spirit of the Puzzle, Yami Yugi. Yugi was initially unaware of his existence and control of Yugi's body, and would involuntarily shift to Yami Yugi at trying times. He lives with his paternal grandfather, Solomon Muto, and his mother. Yugi is part of a close-knit group of best friends: Joey Wheeler, Tristan Taylor, Téa Gardner and Bakura Ryou. Yugi's friends often cheer him on from the sidelines, and Joey has dueled at his side. Yugi has been friends with Téa since childhood and has a romantic crush on her. Téa has been shown to have romantic feelings for both Yugi and Yami Yugi

In the early manga, Yami Yugi starts out as an merciless vigilante. He emerges to deal justice to people who trespass the souls of Yugi and his friends. Although his acts are righteous, he lives up to his namesake of Dark; his Penalty Game punishments are usually very brutal. Later, while living inside Yugi's body, he seems to become more of a caring, nice and friendly disposition, even though he always had that in him, and the friendship of Yugi's friends had helped it be revealed to others. Yami is a strong, powerful and skilled Pharaoh. He attempts to balance darkness and evil in his use of the Shadow Magic from his Millennium Item. Unlike Yami Bakura and Yami Marik, he never uses it for personal gain, instead preferring to help others in need, and to inflict punishment only on people who have severely wronged innocents. He is slightly reserved, as he doesn't reveal his existence to Yugi or his friends. At first Yugi feared Yami Yugi, but after Yugi admits to thinking "another Yugi" exists inside him, his friends become aware of his existence and accepted him as a friend. Even before, Yami is extremely protective of Yugi. Yami's pride is his greatest strength and weakness. Initially he refuses to lose games, without considering the consequences. This has caused him to almost kill Seto Kaiba by attacking his monster while he stood on a ledge and the shockwaves could cause him to fall if Yugi chose to win the Duel (in the Japanese version, Kaiba threatened to commit suicide should he lose). Yugi holds him back from such acts at times, but he is not always successful. Yugi becomes afraid to let Yami out for a while, because of the harm he could cause. During his Duel with Rafael, Yami, albeit with great hesitation, eventually ignored Yugi's objections and used the Seal of Orichalcos rather than lose. Ironically, it was this action and Yami's subsequent corrupt actions during the Duel would lead to one of his few defeats and the capture of Yugi's soul by Dartz. Yami expressed great grief and guilt following the battle due to Yugi's willingly suffering, which were because of Yami's actions. However, he was able to conquer and purge the darkness within himself; which also eradicating the malignant influence of the Orichalcos that remains within him. Learning how to lose became an essential aspect of Yami's journey, which he is tested in the Ceremonial Battle. Yami is also wise and intelligent. He knows all the rules and details of how the game is played, as well as those of all games. He also knows several special game techniques, such as the Double Hit dice roll technique, which, when two dice are used and the one with the weak spin stops first, lets him make the dice that's already spinning hit the one that isn't, allowing him to change its number, which is done by bumping the field with a body part, e.g the elbow. Despite being an ancient pharaoh, Yami is very quick to learning and utilizing modern technology. He, despite initial shock, was able to quickly grasp the concept of holographic technology, how Dueling Arenas and field Power Bonuses worked, was able to use the Dungeon Dice Monsters Help computer to quickly formulate strategies, and was able to use Duel Disks flawlessly on the first try. Yami is also capable of translating the hieroglyphics of the Egyptians, figuring out the riddles or clues and what they mean. Through Yugi and his friends, Yami Yugi learns the values and powers of friendship and unity. After seeing Yugi overcome Marik in his Duel with Joey, who has been possessed, Yami thinks to himself, he learned something from Yugi; that in kindness lies the greatest strength of all; one day Yugi will surpass him in every way.[2] In the Japanese anime, Yami often refers to Yugi as "aibou" ("partner"). There have been instances where he also refers to Yugi as "other me". Being more prideful than Yugi, he uses "ore". As such, he would call Yugi "mou hitori no ore".